Electrical apparatus



Patented Sept. 23, 1930 UNITED STATES PATENT OFFICE HARRY M. WILLIAMS, OF DAYTON, OHIO, ASSIGNOR TO FRIGIDAIRE CORPORATION, OF DAYTON, OHIO, A CORPORATION OF DELAWARE ELECTRICAL APPARATUS No Drawing.

This invention relates'to improvements in electrical contacts and alloys therefor and has for its object to produce a contact which will resist corrosion and deterioration when used for closing and opening electrical circuits,

particularly in circuit-breakers, switches, re-

lays and the like. I

\Vith this object in view, the invention contemplates providing a contact composed of an alloy of zinc and silver in proportions which will withstand repeated and prolonged operation with relatively heavy current without pitting and tarnishing and which is economical for use.

It is well known that the making and breaking of electrical circuits, particularly those carrying considerable current causes the production of arcs which in combination with the oxygen of the air and various impurities such as sulphur and the like present in the air, cause rapid tarnishing and corrosion of the contacts of the device used for breaking the circuit. Such corrosion prevents the subsequent efiective completion of.

the circuit because of the greater resistance which the tarnish film possesses as compared to that of the metal itself; In addition the arc causes localized fusion of the metal which is drawn out into sharp points which proao trude and prevent the broader surfaces from. contacting properly. The contacting of such sharp points against a flat surface under influence of an arc causes the phenomenon of pitting. In the early days of electrical technology, platinum was extensively employed as a contact material, but its high cost precludes its present commercial application for this purpose, while silver which has largely replaced platinum is unsatisfactorysince 1t 4'1 readily tarnishes, due to the formation of a sulphide coating. 7

In accordance with the present invention, an improved contact is prov1ded wh1ch is cheaper than silver and more tarnlsh resist- 4 ing and which is composed of a silver alloy 1928. Serial'No. 267,125.

capable of resisting relatively high temperatures and repeated circuit breaking.

The alloy of which the contact is made comprises up to of'zinc with the balance silver. A particular embodiment of the invention chosen by way of illustration from the range indicated contemplates the use of 20% zinc and 79.9% or substantially 80% of silver. This alloy is appreciably cheaper than pure silver and more tarnish resisting when used for contact purposes. Its freezing point is 780 C. It is light yellow in color, has a density at 25 C. of 9.69 gms. per cm while its tensile strength is roughly .31 lbs. per square inch. Its elongation is 70% in .5 inch; hardness 9.5 scleroscope 420 Brinell, and, when annealed, 37 baby rinell.

It has been found that contacts made of this alloy withstand corrosion and pitting better than contacts made of pure silver. Thus, in comparison with pure silver, contacts made of both materials were run at 32 volts and 5 amperes for 192 hours during which time 320,000 circuit interruptions were made, while in another test both types of contacts were operated at 32 volts and 5 amperes for 49 hours for a total of 82,000 contacts, after which the current was reduced to 3 amperes and the test continued for 54: hours, making 91,000 contacts or circuit interruptions. In both cases the pure silver contact was deeply pitted and corroded, while the contact made of the zinc-silver alloy above described resisted pitting to a far greater extent than silver while it also was substantially less tarnished than the silver.

While the form of embodiment of the invention as herein disclosed, constitutes a preferred form, it is to be understood that other forms might be adopted, all coming within the scope of the claims which follow.

What is claimed is as follows: Y

1. A contact for an electrical circuit breaking mechanism composed of a zinc-silver al- 2. A contact for an electrical circuit breaking mechanism composed of a zinc-silver al- 5 loy, the silver predominating.

3. A tarnish and pitting resisting electrical contact composed of a zinc silver alloy.

4. An electrical contact composed of an al loy comprising up to 30% of zinc'and the balance silver.

5. An electrical contact composed of an alloy of zinc substantially and silver substantially 80%.

In testimony whereof ll hereto aflix my sig- 5 nature.

HARRY M. WILLIAMS. 

